1. Field of the Invention
The present invention relates to a coating composition, more particularly to a coating composition used for the exterior of passenger cars, trains, and other vehicle bodies or for the exteriors of general houses, office buildings, and other buildings, able to resist deposition of dirt, and superior in cleanability by rinsing off the dirt.
2. Description of the Related Art
Broad use is being made of organic polymer paints for the exteriors of passenger cars, trains, and other vehicle bodies and exteriors of general houses, office buildings, and other buildings for the purpose of improving the beauty of the exteriors and protecting the substrate from rust and other corrosion.
Below, coatings formed by organic polymer paints will be referred to as xe2x80x9corganic polymer paint coatingsxe2x80x9d or simply xe2x80x9corganic paint coatingsxe2x80x9d.
Vehicle bodies or buildings formed with the above organic paint coatings on their surface are exposed to the outside air, so dirt floating in the air ends up depositing on the organic paint coatings due to the attraction of static electricity or rainwater as medium. That is, the surface of the vehicle body or building becomes dirty and the external appearance is ruined.
Further, the dirt ends up becoming fixed to the organic paint coating over time and becomes hard to clean off by just rain or rinsing.
Looking at the cleanability of dirt deposited on organic paint coatings, due to the theory of interfacial energy of a plurality of substances in relative contact, it is known that the smaller the water contact angle on the surface of the organic painting coating becomes, that is, the higher the hydrophilicity of the surface of the organic painting coating becomes, the better the cleanability by rinsing off the dirt becomes.
FIG. 1 is a view of the results of the bonding energy of the carbon on the surfaces of substrates having various water contact angles in the air (A) and in the water (W) found experimentally.
Here, the carbon is a substance typifying the dirt. The smaller the bonding energy of the carbon becomes, the easier the dirt is to remove.
In the figure, in water (W), the smaller the water contact angle on the surface of the substrate becomes, that is, the higher the hydrophilicity of the surface of the substrate becomes, the smaller the bonding energy of the carbon becomes and the better the cleanability by rinsing becomes.
Since the water contact angle on the surface of a general organic paint coating used for an automobile or building is a large value of 75xc2x0 to 105xc2x0, the results show that dirt easily deposits on the surface.
As a method for applying a hydrophilic coating material reducing the water contact angle on the surface of a substrate, such as the surface of an organic paint coating, the method of coating an aqueous solution of a polyvinyl alcohol or another hydrophilic organic polymer and the method of coating a dispersion comprised of titanium oxide particles having a photocatalysis function dispersed in a solvent have been developed.
Summarizing the problems to be solved by the invention, the method of coating an aqueous solution of an aqueous organic polymer can improve the cleanability of dirt by rinsing by reducing the water contact angle on the surface of the substrate, but suffers from the problem of durability since the coating dissolves and is washed away due to rain or rinsing.
The method of coating a solution of titanium oxide particles having a photocatalysis function can reduce the water contact angle by breaking down the water by photocatalysis and generating active oxygen and can improve the cleanability, but there is the problem that the paint coating of the substrate is also broken down and deteriorated.
Further, there is the method of adhering a film processed in advance to have a hydrophilic surface on to the surface of the substrate through a adhesive.
This film, however, has to be adhered matching the shape of the substrate. When the substrate is complicated in shape, its adhesion becomes extremely difficult.
The surface being the problem in cleanability is not limited to the above organic paint coating. A similar situation occurs in surfaces of plastic, glass, and other general substrates.
An object of the present invention is to provide a coating composition able to improve the cleanability and durability with respect to the surface of a substrate formed with an organic paint coating or the surface of a metal, plastic, glass, or other general substrate, and another object of the present invention is to provide a coating method using the coating composition.
To achieve the above object, the coating composition of the present invention comprises superfine particles of a metal oxide transparent with respect to visible light and having conductivity and a binder containing a polyvinyl alcohol silylated by an isocyanate silane.
In the coating composition of the present invention, the isocyanate silane is a compound of: 
(wherein, R1, R2, R3, and R4 are each a hydrogen, alkyl group, alkoxy group, or other monovalent substituent group, at least one substituent group among R1, R2, R3, and R4 including an isocyanate group (xe2x80x94Nxe2x95x90Cxe2x95x90O)) and can introduce a silyl group (xe2x80x94SiR1R2R3 (where R1, R2, and R3 are each a hydrogen, alkyl group, alkoxy group, or other monovalent substituent group) into a polyvinyl alcohol ((xe2x80x94CH2xe2x80x94CHOHxe2x80x94)n).
The binder including the above silylated polyvinyl alcohol can provide a surface with a high hydrophilicity, that is, a surface with a small water contact angle, for example, can make the water contact angle about 9 degrees.
Further, the binder containing the polyvinyl alcohol silylated by an isocyanate silane is designed to be resistant to dissolution in water or the rain by the introduction of the silyl group. Due to this, it is possible to improve the durability of the coating layer formed by coating the coating composition of the present invention.
Further, the coating composition of the present invention contains superfine particles of a metal oxide transparent to visible light and having conductivity.
The smaller the conductivity of the surface becomes, that is, the larger the electrical resistance of the surface becomes, the larger the amount of deposition of dirt due to the static electricity of the surface of the substance becomes. The larger the electrical resistance becomes, the longer the charge half life becomes and the easier the deposition of dirt becomes due to static electricity. For example, the charge half life of the surface of a general organic paint coating used in an automobile or building is a long value of 40 to 60 minutes.
To deal with this problem, it is possible to provide a coating layer formed by coating the coating composition of the present invention where the superfine particles of the metal oxide having conductivity enable the electrical resistance to be made smaller and the static electricity to be quickly removed and enable the charge half life of the surface of the coating layer formed by coating the coating composition of the present invention to be greatly shortened.
Therefore, it is possible to reduce the deposition of dirt due to the static electricity of the surface of the substance.
Further, the superfine particles of the metal oxide are transparent to visible light and will not detract from the external appearance of the surface formed with the coating layer.
As explained above, the coating composition of the present invention can be coated on an organic paint coating, metal, plastic, glass, or other substrate to reduce the water contact angle on the surface of the coating layer and simultaneously reduce the electrical resistance of the surface to improve the cleanability, durability, etc.
The coating composition of the present invention preferably contains a metal oxide comprised of at least one of tin oxide and indium oxide.
Further, preferably, the average size of the superfine particles of the metal oxide is in the range of 5 to 15 nm.
Superfine particles or tin oxide, indium oxide, or other metal oxides with an average size in the range of 5 to 15 nm can be made transparent to visible light.
The coating composition of the present invention preferably further contains colloidal silica.
By including colloidal silica, it is possible to improve the hydrophilicity of the surface of the coating layer.
The coating composition of the present invention preferably contains as a solvent water, a glycol ether-based hydrophilic solvent, an alcohol-based hydrophilic solvent, or a mixture thereof.
By diluting the composition by the above solvent, it is possible to obtain a coating composition adjusted in viscosity etc. and made superior in coatability.
To achieve the above object, the coating method of the present invention comprises a step of coating above coating composition on a substrate.